The present invention relates to composite articles of ultrahigh molecular weight polyethylene and melt-flowable polyethylene and a method of forming the same.
Ultrahigh molecular weight polyethylene (UHMWPE) is a high density polyethylene with a molecular weight over 3xc3x97106. Chemically, high density polyethylene (HDPE) and UHMWPE are identical; both are straight-chain linear polymers. The molecular weight of conventional HDPE is rarely above 50,000; whereas, the molecular weight of UHMWPE is commonly above 3xc3x97106 and can be as high as 6xc3x97106.
Ultrahigh molecular weight polyethylene polymers are said to provide a level of abrasion resistance greater than that of any other thermoplastic and also to offer the highest impact toughness of any plastic. These polymers also offer good corrosion resistance and excellent environmental stress-crack resistance, and may be used in food processing situations. These polymers have also been used in bearing pads, prosthetics such as hip socket joints and artificial knees, and in other articles where the wear resistant properties of ultrahigh molecular weight polyethylene is desirable and advantageous.
In order to fasten bearing pads or other articles molded of UHMWPE to a supporting plate such as a fifth wheel or other rigid plate, screw fasteners are commonly used, as shown in U.S. Pat. No. 5,620,770. However, as bearing plates of this type become worn in use, the screws eventually become exposed and can damage the articles that bear against the pads. Efforts have been made to adhere ultrahigh molecular weight polyethylene pads to rigid plates, but this polymeric material is chemically rather inert and bonds to other materials, even HDPE and other polyolefins, only with difficulty. Adhesive bonds between UHMWPE and supporting plates of steel or the like may not be satisfactory when the bearing pads are to be used under circumstances in which the pads must withstand substantial compression, impact and shear forces.
Ultrahigh molecular weight polyethylene articles are difficult to form. Commonly, ultrahigh molecular weight polyethylene is molded in the form of plates or rods, and parts then are machined from these bulk shapes. This is due at least in part to the difficulty that has been experienced in the past in molding ultrahigh molecular weight polyethylene into intricate shapes and forms. Ultrahigh molecular weight polyethylene does not melt and flow well; it gets soft at or near the melting point of HDPE; that is, in the range of 125-130xc2x0 C. Because of its resistance to melt and flow, it is difficult to mold large UHMWPE items or articles having intricate shapes and forms.
Melt-flowable polyethylenes having lower molecular weights, such as HDPE, low density polyethylene (LDPE), and linear low density polyethylene (LLDPE), melt and flow with relative ease by comparison and are more readily molded than UHMWPE. These lower molecular weight polyethylenes are generally characterized as having a unique combination of properties: toughness, high impact strength, low brittleness temperature, flexibility, processibility, chemical resistance, low permeability to water, stability, and outstanding electrical properties. Their low melting point and high chemical stability facilitates processing by conventional techniques, such as injection molding, blow molding, and extrusion.
It would be desirable to form a composite article having a UHMWPE portion for use as a bearing surface, as for snowmobile skis and the like on the one hand, coupled to a melt-flowable polyethylene portion capable of being molded into desired shapes on the other hand, the article being capable of being made by rapid injection molding techniques and providing a strong bond between the portions. Unfortunately, as pointed out above, whereas LLDPE, LDPE and HDPE all melt and flow well, thus enabling them to be formed and shaped while molten, UHMWPE does not melt and flow to any appreciable extent. It would also be desirable to form such composite articles without the disadvantages of prolonged processing times and high energy input required to form similar articles exclusively of ultrahigh molecular weight polyethylene.
The present invention is directed to a method of rapidly forming a composite article having joined ultrahigh molecular weight polyethylene and melt-flowable polyethylene portions. The UHMWPE portion has a surface that is textured with a pattern of ribs. The ribs may vary in shapes and configurations, such as a series of parallel ribs or a plurality of pins of various cross-sectional shapes, e.g., oval, round or rectangles. The ribbed surface of the UHMWPE portion is contacted with molten, melt-flowable polyethylene that melt-bonds or fuses to the ribs of the UHMWPE portion. Using the method of the invention, composite articles can be rapidly formed, as by injection molding. Further, the composite articles of the present invention can be formed without the high energy input and longer processing time heretofore required to form similar articles exclusively of UHMWPE.
In a preferred embodiment of the method of the invention, a UHMWPE portion having a ribbed surface is contacted with molten, melt-flowable polyethylene. The ribs, having thin, easily heated sections, are so configured and the molten, melt-flowable polyethylene is at a sufficient temperature that the temperature of the ribs is raised quickly to a temperature, enabling the polyethylene strongly to melt-bond to the ribbed surface.
In another embodiment, the present invention is directed to composite articles having joined ultrahigh molecular weight polyethylene and melt-flowable polyethylene portions having high bond strength between the portions. The ultrahigh molecular weight polyethylene portion of this composite article includes a surface having ribs extending outwardly from it, such surface and the surface of the ribs of the ultrahigh molecular weight polyethylene portion being melt-bonded to the melt-flowable polyethylene portion. These composite articles are formed using the method of the present invention.
As used herein, the terms xe2x80x9cultrahigh molecular weight polyethylenexe2x80x9d and xe2x80x9cUHMWPExe2x80x9d refer to polyethylene that flows only with great difficulty when heated and that has a molecular weight of about 3xc3x97106, whereas the term xe2x80x9cmelt-flowable polyethylenexe2x80x9d refers to meltable polyethylenes of lower molecular weight than UHWMPE and includes high density polyethylene (HDPE), linear low density polyethylene (LLDPE), and low density polyethylene (LDPE).
The exact chemical and physical nature of the bond formed between the two portions of an article according to the invention has not been determined. Although we do not wish to be bound by any specific theory, it appears that the surfaces are fused or melt-bonded such that the molecules of the UHMWPE portion and those of the melt-flowable polyethylene portion are intermingled at and across the surfaces of the two portions. Thus as used herein, the terms xe2x80x9cmelt-bondxe2x80x9d or xe2x80x9cmelt bondedxe2x80x9d refer to and mean that the bond strength between the two portions is equal to or greater than the internal tensile strength of either portion.